1. Field of the Invention
This invention relates in general to structures and fabrication methods for providing an internal shield to protect depletion regions of semiconductor devices from discharge by ionizing radiation or particles, or other spurious carriers and, more particularly, to improved means and methods for shielding the depletion region of dynamic charge storage devices to prevent undesirable discharge thereon by ionizing radiation or particles absorbed within the device substrate or by carriers injected or pumped from nearby regions.
2. Background Art
Charge storage is a frequently used technique in semiconductor devices and integrated circuits. Examples of devices whose operation depends critically on charge storage are dynamic random access memories, bucket brigade shift registers, and charge coupled imaging devices. Many other kinds of semiconductor devices and integrated circuits also use charge storage. Depletion region charge storage may be accomplished using a physical P-N junction formed by abutting doped P and N regions. However, depletion regions may also be induced, as for example, by means of an MOS capacitor or the like. If a sufficiently large voltage is applied across an MOS capacitor or equivalent, a depletion region akin to that found in a P-N junction is created between the semiconductor surface and the bulk. Such induced depletion regions can equally well be used for charge storage devices. As employed herein, reference to storage of charge on junctions is intended to include charge stored in depletion regions whether formed by a permanently doped P-N junction or field induced or by formed by any other means.
A significant problem that arises in connection with the use of charge storage is that the stored charge is subject to being discharged by minority carriers or electron-hole carrier pairs which reach the depletion region. These free carriers may arise from a number of sources, such as, generation within the junction region, injection from another nearby junction, or diffusion from outside the junction region, i.e., from within the bulk of the underlying semiconductor substrate. The result of the discharge is that the information represented by the stored charge decays. If discharge is severe, the information stored in the form of charge may be lost entirely. As a consequence, the charge representing the stored information must usually be periodically refreshed. The greater the number of free carriers created within or reaching the charge storage depletion region per unit time, the more frequently this stored information must be refreshed, that is, the lost charge replaced.
Thermal carrier generation within the depletion region can dissipate the stored charge. Thermal carrier generation and carrier lifetime are related. In order to minimize thermal carrier generation in the depletion region, the carrier lifetime must be made as long as possible. Thus, great effort is expended to obtain long lifetime material in which to form the depletion regions used for charge storage. However, the longer the lifetime, the greater the probability that carriers from elsewhere in the device, e.g., carriers generated in the underlying substrate or injected from nearby junctions, will diffuse into the depletion region and discharge the stored information. Thus, conflicting requirements are encountered when trying to reduce all sources of carriers which might contribute to dissipating the stored charge and thereby requiring more frequent refresh.
Ionizing radiation and particles absorbed within a semiconductor material produce free carriers by ionization. Generally, the depletion region of a charge storage device is relatively thin so that few ionizing events occur directly therein. Further, the depletion region is often located near a device surface, so that only comparatively low energy radiation or particles are likely to be absorbed there. These low energy particles can be easily filtered out by surface protection layers and so are readily avoided. However, the more energetic particles or radiation will pass through the surface layers and be absorbed in the bulk of the substrate. The carrier pairs freed by ionizing events occurring in the substrate can readily diffuse to the depletion region if the semiconductor substrate has a high lifetime. It has been found that these bulk generated carriers are a significant cause of low storage times in many types of charge storage devices.
One method for reducing the discharging effect of in-diffusing carriers is to reduce the bulk lifetime so that more of the bulk generated carriers recombine before reaching the depletion region of the storage device. Two methods of carrier lifetime control which are commonly used in silicon devices, for example, are electron bombardment and gold doping. Electron bombardment is believed to reduce lifetime by introducing defects in the crystal lattice. A disadvantage of using bombardment induced lattice defects for lifetime control is that such defects anneal out during subsequent high temperature processing steps. Gold doping is another means of controlling lifetime. However, gold doping is incapable of the necessary spatial resolution. For example, gold diffuses so rapidly in silicon at ordinary process temperatures that it travels throughout the entire wafer thickness in a very brief time. Further, the amount of lifetime reduction that can be obtained by these methods is often insufficient to prompt recombination of all in-diffusing carriers.
Another method which has been suggested for providing lifetime control structures is to start with a low lifetime substrate and attempt to grow thereon a high lifetime surface layer in which the charge storage devices would be fabricated. However, this has not proved practical because of the tendency of the substrate to contaminate or create defects in the epilayer, thereby reducing its lifetime.
More recently, argon has been investigated as a means of obtaining localized reduction in lifetime. However, the lifetime reductions from ion implanted argon appear to be primarily related to physical damage to the crystal lattice. As with electron bombardment, the effect is significantly annealed by post implant heating. This is a substantial disadvantage since practical device fabricated methods generally require that the treated wafer be heated to high temperatures during subsequent fabrication steps.
Thus, a need still exists for means and methods for shielding charge storage devices from injected or bulk generated free carriers. Accordingly, it is an object of the present invention to provide improved means and methods for shielding charge storage junctions from bulk generated or otherwise injected carriers.
It is a further object of the present invention to provide improved means and methods for providing high lifetime regions for construction of charge storage devices, while simultaneously providing adjacent low lifetime regions which act as a shield against indiffusion of bulk generated or otherwise injected carriers.
It is an additional object of the present invention to provide improved means and methods for controlling the location, lateral extent, and thickness of such a carrier shield.
It is a further object of the present invention to provide improved means and methods for achieving a very high density of lifetime killing centers in such a carrier shield.
It is an additional object of the present invention to provide means and methods for increasing the density of lifetime killing centers within such a carrier shield to values not previously obtained in the prior art.
It is an additional object of the present invention to provide improved means and methods for obtaining a free carrier shield using impurities which react chemically with the semiconductor substrate to form stable recombination centers.
It is a further object of the present invention to provide an improved means and method for obtaining a free carrier shield whose effectiveness is not substantially annealed by heating.